In medical applications, x-ray radiation is conventionally produced within an evacuated tube. Electrons are generated by a source, such as a heated filament, and accelerated by a high voltage applied to an accelerating electrode. The accelerated electrons are directed toward and impact on a metal target, typically tungsten or molybdenum, resulting in the production of x-ray radiation. Typically, the peak accelerating voltage (kVp) is at least 15,000 volts and can be up to ten times higher. The accelerating voltage employed depends upon the diagnostic or therapeutic use being made of the x-ray radiation. The x-ray radiation increases in penetrating power with an increase in the accelerating voltage applied to the x-ray source.
In diagnostic applications of x-rays, the amount of radiation that passes through an object being analyzed with x-rays determines the contrast of a radiographic image on a fluoroscopic screen or photographic film. In some applications, moderate variations in x-ray penetrating power due to variations in the accelerating voltage applied to the x-ray source do not significantly affect x-ray analysis. For example, small variations in the contrast of photographic x-ray images of broken bones do not affect the ability of a radiologist and/or orthopedist to determine the location and nature of a bone fracture. However, in other applications of x-ray imaging, the contrast of a photographic x-ray image can be critical to accurate diagnosis of a disease or disorder. Two important examples where radiographic contrast control is of substantial or critical importance are mammography and coronary angiography. In those applications, variations in image contrast caused by even moderate variations in accelerating voltage can result in erroneous diagnoses. Therefore, precise measurement of the accelerating voltage applied to an x-ray tube is essential to proper diagnosis and treatment of important, potentially life-threatening diseases and disorders.
Measurement of the relatively high voltages applied to x-ray tubes used as x-ray radiation sources in radiological equipment presents numerous technological problems. Direct measurement of such high voltages is, in principle, straightforward for constant potentials and low frequency accelerating voltages. In the early development of x-ray equipment, power supplies for producing such high voltages were relatively large and frequently encased in a liquid dielectric medium, such as oil. Nevertheless, the output voltage terminals of the accelerating voltage power supply were accessible. In order to measure the accelerating voltage applied to an x-ray tube, a high tension voltage divider was connected across the output terminals of the accelerating voltage power supply so that a portion of the high voltage could be measured. The accelerating voltage could be calculated based upon the voltage dividing ratio of the voltage divider. These voltage dividers function well for the direct current and the low frequency components of the accelerating voltage kVp produced by older x-ray equipment. However, modern x-ray equipment produces accelerating voltages with high frequency components and complex waveforms that conventional voltage dividers cannot accurately measure without specific calibration of the dividers.
Typically, medical x-ray equipment includes a voltage control for varying the accelerating voltage so that the equipment can be used in various applications. The equipment usually includes an indicator, such as a dial or a meter, for indicating the accelerating voltage. The indicator is not directly connected to the accelerating voltage produced by the high voltage power supply but is actually connected to measure a lower voltage input to the accelerating voltage power supply. Historically, the accelerating voltage indicator was calibrated by connecting a voltage divider directly to the accelerating voltage power supply, setting the voltage control to a number of positions, measuring the actual accelerating voltage across the voltage divider for each position of the voltage control, and adjusting the indicator as necessary. The voltage divider is not present during medical use of the equipment so that a radiological technician depends upon the accuracy of the calibration of the indicator when setting the accelerating voltage for x-ray imaging or treatment. With the passage of time, the indicator can become inaccurate and periodic recalibrations are required to restore the accuracy of the indicator. In fact, governmental regulatory bodies frequently require periodic recalibration of x-ray equipment to maintain minimum standards of health safety.
Although power supplies in older x-ray equipment typically directly produced the pulsating, i.e., with some degree of ripple, direct current accelerating voltages using transformers, rectifiers, and capacitive filters, modern x-ray accelerating voltage power supplies are more complex. Modern x-ray equipment power supplies may employ toroidal transformers and inverters as well as rectifiers operating at various frequencies and with pulsed signals having complex waveforms to produce the accelerating voltage. Many parts of the modern power supplies are encapsulated in resins and do not permit access for the connection of a high tension voltage divider for calibrating the accelerating voltage indicator of the equipment.
Non-invasive, i.e., without direct electrical connection, techniques are and may have to be employed to calibrate the accelerating voltage indicators of modern x-ray equipment. One known technique employs two or more metal foils of different thicknesses, generally arranged serially with respective x-ray radiation detectors. In most cases the foils are made of the same materials. The different thicknesses of the foils cause different modifications of incident x-ray radiation that reaches the detectors. The relative intensities of the radiation that penetrates the respective foils is measured and compared, sometimes in a complex mathematical relationship, to determine an empirical correlation between differences in the radiation intensities measured by the respective detectors and accelerating voltage. Convenient methods of evaluating the measured radiation intensities produce results that depend upon the waveform of the accelerating voltage and, therefore, are not universally applicable to all x-ray equipment. Depending upon the voltage applied to the x-ray tube, the accuracy of voltage measurements using single foils ranges from five to ten percent, for example, .+-.2 kV when kVp is 20 kV.
An improvement in measurement accuracy as compared to the use of foils of the same material, at least for measuring a single accelerating voltage, can be achieved when filters of different materials are used. Any of the elements having atomic numbers between 40 and 60 may be used as one of the filters. A second filter having a much lower atomic number and an x-ray absorption characteristic matched to that of the first filter over part of a radiation energy range is used. The single acceleration voltage calibration point is unique to the specific elements used in the filters. In one commercially available filter pair, cadmium and aluminum are used in tandem with a photodiode detector. The photodiode is sensitive to x-rays that penetrate the filters. As the voltage applied to the x-ray tube is increased, significantly different x-ray penetration of the cadmium and aluminum filters occurs, as indicated by the photodiode detector, once the peak accelerating voltage, kVp, exceeds 26.5 kV. Thus, the cadmium and aluminum filter pair provides a precise indication of a single accelerating voltage. Other pairs of filter materials can be used to identify additional individual accelerating voltages precisely although the use of many such filter pairs to detect many accelerating voltages is cumbersome.
Improved precision in accelerating voltage measurement can be made using an ionization spectrometer that measures the shape of the x-ray spectrum near the end point energy. The spectrum of electromagnetic energy produced by an x-ray tube has two recognized components. First, the x-ray spectrum includes sharp "lines" of relatively intense x-ray radiation at wavelengths that are characteristic of and well established for various x-ray tube target materials. These radiation components, typically generally identified as K.alpha. and K.beta. lines, result from the transfer of energy from accelerated electrons to atoms of the target, followed by energy transitions between inner shell electron energy states of target atoms that produce x-ray radiation. The energy transitions between well defined energy levels account for the specific energies of the line components of the radiated x-ray spectrum.
Second, in addition to the sharp lines in the x-ray spectrum, a more broadly distributed component of continuous x-ray radiation is also produced. This radiation, the so-called Bremsstrahlung, results from the scattering of accelerated electrons by the target accompanied by emission of x-ray radiation having energies equal to the energies given up by the electrons in the scattering process. The energies given up are not confined to discrete energies so that a broad x-ray energy distribution, i.e., a continuous x-ray spectrum, is produced. The maximum energy loss that can occur in the electron scattering process occurs when all of the kinetic energy of an accelerated electron is lost and is converted to x-ray radiation. Since the kinetic energy of an accelerated electron equals the electronic charge, e, of the electron multiplied by the accelerating voltage, that total kinetic energy loss produces the highest energy x-rays within the continuous component of the x-ray spectrum. That energy, which may be measured in terms of the maximum frequency or the minimum wavelength of the x-ray radiation, is referred to as the end point energy because it is the upper energy limit of, i.e., end point of, the continuous component of the x-ray radiation.
In an ionization spectrometer, the shape of the x-ray spectrum near the end point energy is measured by analyzing charge pulses produced in a crystalline material, such as intrinsic germanium, in response to incident x-ray radiation. However, despite the improved voltage measurement accuracy achieved with an ionization spectrometer as compared to the use of foils and filters, ionization spectrometer measurements can only be made while a relatively low electron beam current flows in the x-ray tube. Higher currents increase the quantity of x-ray radiation produced and cause overlapping charge pulses in the crystal that cannot be reliably analyzed. Since the spectrometer can only analyze one charge pulse at a time, accelerating voltage measurement using an ionization spectrometer takes a relatively long time. The measured results must be extrapolated for practical electron beam currents, and errors may be introduced in the extrapolation if the accelerating voltage waveform is current-dependent.
Accordingly, it would be desirable to provide a method and apparatus for accurately measuring the accelerating voltage applied to an x-ray tube. Most preferably, the apparatus is portable and the method is simple so that calibration of existing clinical x-ray equipment can be carried out at the site of the equipment, without modification of the equipment, by a technician rather than a scientist, and without undue interruption in the medical use of the equipment.